1. Field of the Invention
The invention of the present disclosure relates to switching voltage regulators and more particularly to DC-DC converters in a computer system.
2. Art Background
The power supplies in a computer system are designed to meet the specific power requirements of the integrated circuit chips (ICs) that are the components of the system. The nominal operating voltages for the ICs are typically known because most ICs are manufactured to meet industry standards for device operation. For example, the nominal supply voltage for transistor-transistor logic (TTL) devices is 5.0 volts while the nominal supply voltage for complementary metal oxide semiconductor (CMOS) devices is 3.3 volts.
A power supply will ideally deliver the nominal voltage levels with perfect assurance and precision, but power supplies are typically inaccurate due to a number of factors. A typical range of assurance for a power supply is plus or minus five percent. Accordingly, most ICs are designed to operate within a range of plus or minus five percent of the nominal voltage. However, some ICs are less tolerant of power supply inaccuracies, and some ICs may require a nominal operating voltage other than the standard TTL and CMOS voltages. The operating voltage of an IC having either one or both of these characteristics can be supplied by a DC-DC converter that converts the DC output of the power supply into the desired DC operating voltage.
DC-DC converters are typically switching voltage regulators, which are more efficient than linear regulators. The need for efficiency is emphasized when the DC-DC converter is to be used to supply voltage to a single IC, which could be the processor of the computer system. If too much power is dissipated while the DC-DC converter is operating, heat sinks will be needed and the footprint of the DC-DC converter will be increased. This is especially undesirable when the amount of available board space is limited. Thus, maximum efficiency is one goal of DC-DC converter design. Unfortunately, typical prior art designs fail to maximize efficiency because switching losses associated with switching the power switching transistor on and off are often not addressed. These switching losses are often magnified in DC-DC converter designs for converting TTL to CMOS voltages because such designs typically implement a p-channel switching transistor, which is often more lossy than n-channel switching transistors.
Another problem in DC-DC converter design are load transients. Some ICs, such as processors, are capable of entering a sleep mode wherein the IC draws little or no current. When the IC exits the sleep mode, the load that the IC presents to the DC-DC converter increases dramatically, and the IC goes from drawing little or no current to drawing its normal current. Such transients must be addressed.
A further consideration in DC-DC converter design is cost. The precision of a switching voltage regulator is primarily dependent on the accuracy of the reference voltage used by the switching regulator. High precision switching regulator ICs are commercially available, but implementing them in a DC-DC converter design can be cost-prohibitive, especially when the DC-DC converter is to be used to supply only a single IC. Therefore, a low cost, high precision DC-DC converter design that implements a low precision switching regulator is desired. As will be described, the method and apparatus of the present invention provide a high-precision DC-DC converter having improved efficiency at a low cost.